Ever been on a large group campout for scouts/church/school? At night, when it gets particularly dark and you are trying to stumble your way back to the kitchen area, you usually end up tripping on the various guylines for tents and falling, bringing down yourself while angering the person inhabiting
that particular tent. Glow in the dark toys and other novelties are rather common, so it shouldn't be too hard to make:

Glow-In-The-Dark tent cord! Bundle tents with glowing guylines to help the fairly common situation of "invisible" string. Put up your tent, shine it with your flashlight, and for the duration of the evening, your tent is protected from damage as well as the poor unsuspecting sap who just needs to get the rest of his gear out of his car.

I had to laugh at this one. I've just arrived back from a weekend camping with 50 women, and I would have loved a glow in the dark tent cord to tie around a certain tree which stood between the ablution block and my camp site. I had several intimate and painful moments in the dark with that tree.

Rechargable "glow-in-the-dark" materials lose most of their glow fairly soon after darkness. If you have a quick transition (like turning out the lights in a room), you can get a useful afterglow, but outdoors, where night falls slowly, the chords won't be very useful once night has actually arrived.

Having each tent cord dipped in a container of
methylated spirits and then lighting the cord should do it.
All night long, the meths will creep up the cord by
capillary action but will burn at a low enough
temperature so as not to burn the cord itself.

Little flashing LEDs at the peg, at the join to the tent, and in the middle of the guy line should do it. They could be controlled by an light sensitive resistor on the tents apex. They would work a lot like those lights on top of chimneys/antennae that discourage aeroplanes from hitting them